But he insisted that, as a former PM, “everything you do becomes a big distraction and diversion”. He hinted strongly his views differ from his successor on key areas and he could not speak out on them without embarrassing her.

He added: “I support Theresa May. I think she’s got off to a cracking start.

“Obviously I have my own views about certain issues. That’s really the point. I want Witney to have an MP that can play a full role in parliamentary and political life in a way I would find very difficult, if not impossible.”

He said his decision “has got nothing to do with any one individual issue”.

But he also pointedly refused to give Mrs May his full backing for her push to bring back ­grammar schools. Instead he merely said the plans had “some merit”.

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A friend and close confidante of the ex-PM said: “Of course he would struggle to vote to bring back grammar schools, and his positive views about the EU are also well known.

That all creates problems which he doesn’t want to create. The more he thought about it over the summer, the more he saw this and decided he needed to go.”

Another friend added: “He doesn’t want to be a back-seat driver. He feels Theresa needs to be able to do it on her own terms, so it’s more loyal for him to step down now.”

Mr Cameron, who turns 50 next month, said he still wanted to “contribute in terms of public service”, a possible nod to a major international role with the UN.

Highly paid consultancy work is also a possibility as he admitted he would “have to start building a life outside of Westminster”.

The realities of modern politics make it very difficult to continue on the backbenches

Here is the full text of David Cameron's statement announcing he is standing down as an MP:

"I spoke this morning to the chairman of the West Oxfordshire Conservative Association and I have just come from a meeting with my constituency agent and other members of staff.

"Having fully considered my position over the summer, I have decided that I am going to stand down as the Member of Parliament for Witney.

"There will now be a by-election and I will do everything that I can to help the Conservative candidate win that election.

"In my view, the circumstances of my resignation as prime minister and the realities of modern politics make it very difficult to continue on the backbenches without the risk of becoming a diversion to the important decisions that lie ahead for my successor in Downing Street and the Government.

"I fully support Theresa May and have every confidence that Britain will thrive under her strong leadership.

"It has been an honour to serve the people of Witney and West Oxfordshire for the last 15 years.

"I want to thank them for making the job of representing them such a pleasure and a privilege.

"I would like to thank the party workers, volunteers, councillors and supporters for all they have done to help me.

"Our house in Dean is the place Samantha, my children and I call home, and that will not change.

"I will continue to support the local causes and charities that mean so much to me and many others in this beautiful and enterprising part of our country.

"I now look forward to a life outside of Westminster, but hope to continue to play a part in public service and to make a real and useful contribution to the country I love."

Ex-Business Minister and long-standing ally Nick Boles, who is gay, said: “Grateful to David Cameron for many things but most of all for giving me the chance to know the joy of marriage. Thank you boss.”

Veteran former Cabinet minister Ken Clarke dubbed Mr Cameron’s resignation “a great pity”, adding: “The House of Commons really does benefit by having some senior people in it staying on in politics”.

Labour leader Jeremy Corbyn said he wished Mr Cameron “all the best for the future”, adding he got on well with him on a “human level”.

Labour Party Leader Jeremy Corbyn wished David Cameron the best of luck for the future

Lib Dem leader Tim Farron added: “I didn’t agree with him on very many issues, but I always appreciated his personal courtesy, humour and thoughtfulness.”

As a former PM, Mr Cameron will now get a car, a police driver and round-the-clock Special Branch protection.

He will also get a Public Duties Cost Allowance — up to a maximum of £115,000 a year — to “meet the continuing additional costs of their special position in public life”.

He is also entitled to a ministerial pension worth around £20,000 from 65 as well as a gold-plated backbench MPs’ pension of around £26,000 a year.

Cam's highs and lows

SEPTEMBER 1988: Cameron started his political career in the Conservative research department before taking up a series of special advisory posts.APRIL 2000: He was ­chosen as Conservative candidate for the ultra-safe Witney constituency in Oxfordshire.JUNE 2001: Cameron was elected as MP and served on the Commons Home Affairs Select Committee until 2003.OCTOBER 2005: Voting begins for leader of the Conservative Party after Michael Howard stood down. Cameron announced winner in December with a mandate to modernise.MAY 2010: Cameron became Prime Minister when the Conservatives formed a coalition with the Liberal Democrats. Cameron was the youngest Prime Minister since 1812 at the age of 43.JULY 2013: Same-sex marriage was legalised in the UK, often seen as a highlight of Cameron’s career.SEPTEMBER 2014: Scotland voted to remain in the United Kingdom in a referendum after Cameron campaigned for result.MAY 2015: The Conservatives won the General Election with a surprise majority. One of their main manifesto pledges was a referendum on the UK staying in the EU.SEPTEMBER 2015: Cameron faced public ridicule after an unauthorised biography claimed he inserted a “private part of his anatomy” into a dead pig’s mouth while at Oxford. He denied it.APRIL 2016: Leaked documents from Panama revealed he profited from an offshore trust belonging to his father. Cameron had repeatedly spoken out against tax evasion.JUNE 2016: After campaigning for the UK to remain, Cameron resigned as PM when the country voted in favour of Brexit.SEPTEMBER 2016: Cameron resigned as an MP, triggering a by-election in his Oxfordshire seat.